1. Field of the Invention
The present invention relates to radio frequency identification (RFID) interrogators and transponders, and more particularly, to a novel communication architecture for an RFID interrogator system.
2. Description of Related Art
In the automatic data identification industry, the use of RFID transponders (also known as RFID tags) has grown in prominence as a way to track data regarding an object to which the RFID transponder is affixed. An RFID transponder generally includes a semiconductor memory in which digital information may be stored, such as an electrically erasable, programmable read-only memory (EEPROMs) or similar electronic memory device. Under a technique referred to as xe2x80x9cbackscatter modulation,xe2x80x9d the RFID transponders transmit stored data by reflecting varying amounts of an electromagnetic field provided by an RFID interrogator by modifying their antenna matching impedances. The RFID transponders can therefore operate independently of the frequency of the energizing field, and as a result, the interrogator may operate at multiple frequencies so as to avoid radio frequency (RF) interference, such as utilizing frequency hopping spread spectrum modulation techniques. The RFID transponders may either extract their power from the electromagnetic field provided by the interrogator, or may include their own power source.
Since RFID transponders do not include a radio transceiver, they can be manufactured in very small, light weight and inexpensive units. RFID transponders that extract their power from the interrogating electromagnetic field are particularly cost effective since they lack a power source. In view of these advantages, RFID transponders can be used in many types of applications in which it is desirable to track information regarding a moving or inaccessible object. One such application is to affix RFID transponders to work pieces moving along a conveyor belt of an assembly line. The RFID transponders would contain stored information regarding the particular assembly requirements for the work piece to enable automated equipment to operate on the work piece and perform certain tasks particular to the unique work piece requirements. This way, products having different assembly requirements can be sent down the same assembly line without having to modify the assembly line for each unique requirement. Another application for RFID systems is to collect information from a moving motor vehicle, such as for vehicle toll collection.
The backscatter-modulated signal reflected by the RFID transponder may contain relatively low power and dynamic range. Therefore, it is important for the RFID interrogator to minimize the noise in both the transmitted and received signal paths in order to achieve an acceptable read range and error rate of the received data. The RFID interrogator transmits full power while receiving data in accordance with the backscatter modulation technique. As a result of the simultaneous transmitting and receiving, a portion of the transmitted signal can leak into the received signal path, providing a significant source of noise to the received signal. Moreover, there may be a small frequency offset between the transmitting and receiving signal frequencies, further producing noise and interference of the received signal. Another noise source comes from demodulator which mixes the received signal with the carrier frequency in order to downcovert the received signal to baseband or an intermediate frequency signal. The mixing stage can produce signal components that reflect back into the carrier, or that can produce absolute and/or additive phase noise.
Accordingly, it would be very desirable to provide an RFID interrogator having a receiver/transmitter architecture that attenuates these and other inherent noise sources in order to achieve increased read range and reduced error rate of the received data.
In accordance with the teachings of the present invention, an RFID interrogator is provided having a low-noise radio receiving/transmitting system. The RFID interrogator further comprises a microcontroller module adapted to provide high level commands to the interrogator, a DSP module for processing received/transmitted data and controlling radio operations, and a radio module for transmitting and receiving RF signals to/from an RFID transponder.
A first embodiment of the RFID interrogator comprises an RF carrier source providing a carrier signal, a processor providing an information signal, and plural modulation stages coupled to the RF carrier source for modulating the information signal onto the carrier signal using on/off keying modulation. The plural modulation stages are controlled in unison by control signals the said processor. A first one of the plural modulation stages comprises a mixer coupled to the RF carrier source and the processor. The mixer combines the carrier signal with the information signal to provide a modulated signal having on and off states in correspondence with the information signal. A second one of the plural modulation stages comprises an amplifier coupled to the mixer. The amplifier is responsive to control signals from the DSP to amplify the modulated signal only during on states of the modulated signal. A third one of the plural modulation stages comprises a switch having a first position coupled to a transmission path and a second position to a resistor termination. The switch is responsive to the control signals to couple the modulated signal to the transmission path during the on states of the modulated signal and to couple the modulated signal to the resistor termination during the off states of the modulated signal. The plural modulation stages provide dynamic range of greater than 25 dB between respective on and off states of the modulated signal.
A second embodiment of the RFID interrogator comprises an RF carrier source providing a carrier signal, a modulator receiving the carrier source and modulating an information signal thereon to provide a modulated RF signal, and an amplification stage coupled to the modulator. The amplification stage splits the modulated RF signal into first and second components, and amplifies the first and second components separately. The first and second components are thereafter recombined. The split operation of the amplification stage provides a better match with the downstream transmission path than an individual amplifier. More specifically, the amplification stage comprises a first quadrature hybrid adapted to split the modulated RF signal into the first and second components having a 90xc2x0 phase difference therebetween, first and second power amplifiers receiving the first and second components respectively and providing amplification thereto, and a second quadrature hybrid adapted to recombine the amplified first and second components, respectively. The first and second quadrature hybrid further comprise respective isolated signal paths for communicating reflected power from the first and second power amplifiers therefrom, and resistor terminations coupled to the respective isolated signal paths. The first and second power amplifiers are each driven at a saturation level in order to minimize amplitude fluctuations that could leak into the receiver path.
A third embodiment of the RFID interrogator comprises an RF carrier source providing a carrier signal, a demodulator combining a received signal with the RF carrier to provide a baseband signal, and an attenuator coupled to the demodulator for absorbing high frequency components of the baseband signal. The baseband signal further comprises in phase (I) and quadrature phase (Q) components. The attenuator further comprises a high-pass filter and a resistor termination. The high frequency components of the baseband signal pass through the high-pass filter to the resistor termination, where they are absorbed. The RFID interrogator further comprises a circulator adapted to pass the carrier signal in a first direction therethrough and the received signal in a second direction therethrough opposite to the first direction. The circulator is coupled to the demodulator. A directional coupler is disposed closely adjacent to the circulator in order to minimize noise due to leakage by the circulator. The directional coupler passes the carrier signal to the demodulator.